Compounds containing perfluoropolyether moieties generally have very small surface free energy to have water and oil repellency, chemical resistance, lubricity, releasing property, and antifouling property. Making use of these properties, they are widely used as, for example, treatment agents to make paper or fiber water- and oil- repellent, and foul-resistant, lubricants for magnetic storage media, oil repellent agents for precision apparatuses, releasing agents, cosmetics, and protective films.
These properties, on the other hand, mean that such a fluorine-containing compound does not bond to other materials. Even if it can be applied to a substrate, it hardly forms a coating bonded strongly to the substrate.
Meanwhile, it is well known that an organic compound can be bonded to a surface of glass or cloth via a silane coupling agent. The silane coupling agent has an organic functional group and a reactive silyl group, usually an alkoxy silyl group. The alkoxy silyl groups are autocondensed in the presence of moisture to form a siloxane coating film. At the same time, the alkoxy silyl group chemically bonds to glass or metal surface to form a durable film. The silane coupling agents are thus widely used as coating agents for various substrates.
Japanese Patent Application Laid-Open No. S58-167597 discloses a fluoroaminosilane compound represented by the following formula (8):

wherein R1 and R2 are alkyl groups having 1 to 4 carbon atoms, Q1 is CH2CH2CH2 or CH2CH2NHCH2CH2CH2, h is an integer of from 1 to 4, and i is 2 or 3.
The perfluoropolyether moiety of the compound, however, is relatively short, i.e., a dimer to pentamer of hexafluoropropylene oxide, so that the aforesaid characteristics of the perfluoropolyether are not significant.
Japanese Patent Application Laid-Open No. S58-122979 discloses a water- and oil-repellent agent for glass surface. The agent is represented by the following formula:
wherein Rf1 is a polyfluoroalkyl group having 1 to 20 carbon atoms and may contain one or more of ether group, R3 is a hydrogen atom or a lower alkyl group, A is an alkylene group, X1 is —CON(R4)-Q- or SO2N(R4)-Q-, wherein R4 is a lower alkyl group and Q is a divalent organic group, Z is a lower alkyl group, Y is a halogen atom, alkoxy group or R5COO—, wherein R5 is a hydrogen atom or a lower alkyl group, s is 0 or 1, t is an integer of from 1 to 3, u is an integer of from 0 to 2. The fluorinated group of this compound has carbon atoms not more than 20, so that effects of the fluorinated group are not sufficient.
Recently, needs for technology to attain foul-resistance and easy removal of fouling are increased, for example, to make maintenance free windows of high buildings and fingerprint-proof displays. Materials which meet these requirements are desired.
Japanese Patent Application Laid-Open No. 9-258003 discloses a lens which has an antifouling layer prepared from a fluorine-containing silane compound. The fluorine-containing silane compound has both properties of aforesaid perfluoropolyether moiety and silane coupling agent. The compound has relatively larger amount of hydrolyzable groups per molecule. However, a coating prepared from the compound does not strongly bond to a substrate and is not sufficiently durable. When applied to a lens, desired properties are not maintained for a long time.
An antireflection layer, which is generally formed on visual tools and equipments, is prone to be fouled with fingerprints, sweat, saliva, or hair dressing. The fouling causes change in surface reflectance. Further, the fouling stands out as white object to be more eminent than those on an ordinary transparent object. Therefore, an antireflection layer is desired which has excellent antifouling property, and is easy to clean and durable.
Japanese Patent Publication of Examined Application No. 6-5324 discloses an antireflection film having improved antifouling property. The layer is formed on an antireflective monolayer or multilayer mainly composed of silicon dioxide deposited by PVD method. The layer is composed of organopolysiloxane polymer or perfluoroalkyl group-containing polymer.
Fouling by human secretion and fingerprint, however, is difficult to wipe off from the layer and tends to extend to form a thin oily film. When it is rubbed strongly, the antireflection film itself is damaged.
Japanese Patent Application Laid-Open No. 11-29585 discloses an antireflection film having an antifouling layer prepared from a perfluoropolyether-modified aminosilane represented by the following formula (10):
wherein X3 is a hydrolyzable group, R5 is a lower alkyl group, R6 is a hydrogen atom or a lower alkyl group, Q2 is CH2CH2CH2 or CH2CH2NHCH2CH2CH2, e is an integer of from 6 to 50, f is 2 or 3, and c and d are integers of from 1 to 3.
The aforesaid aminosilane takes a relatively long time for curing due to a relatively small wt % of the hydrolyzable group based on its molecular weight. Further, the cured coating layer does not bond strong enough to a substrate.
Japanese Patent Application Laid-Open No. 2001-188102 discloses an antireflection film having an antifouling layer prepared from a perfluoropolyether moiety-containing silane coupling agent represented by the following formula (11):Rf2—(OC3F6)k—O—(CF2)j—(CH2)e—O—(CH2)r—Si(OR7)3   (11)
wherein Rf2 is a linear or branched perfluoroalkyl group having 1 to 16 carbon atoms, R7 is a C1-10 alkyl group, k is an integer of from 1 to 50, r is an integer of from 0 to 6, j is an integer of from 0 to 3, and 1 is an integer of from 0 to 3, with j+1 being larger than 0 and 6 or smaller.
The aforesaid perfluoropolyether moiety-containing silane coupling agent also takes a relatively long time for curing due to a relatively small wt % of the hydrolyzable group based on its molecular weight. Further, the cured coating layer does not bond strongly to a substrate.
Japanese Patent Application Laid-Open No. 2002-348370 and No. 2003-113244 disclose a silane coupling agent which contains a perfluoropolyether moiety and many hydrolyzable groups at one end. The silane coupling agent is described to have a good film forming property.
Japanese Patent Application Laid-Open No. 2003-238577 discloses a silane coupling agent which contains a perfluoropolyether moiety represented by the following formula (12):

wherein Rf is a divalent linear perfluoropolyether group, R is a C1-4 alkyl or phenyl group, X is a hydrolyzable group, p is an integer of from 0 to 2, q is an integer of from 1 to 5, and a is 2 or 3.
The above silane coupling agent of the formula (12) has two or three hydrolyzable groups per molecule at both ends to form a coating layer which bonds strongly to a substrate. The layer is resistant to fouling. Once it is fouled, however, the fouling is not so easy to wipe off. Ease of cleaning by wiping off depends not only on a coating surface's capability of releasing fouling but also slickness of the surface. Supposing that there are two coatings having equivalent capability of releasing fouling but different surface slickness, the slicker one is easier to clean than the other. Supposing that there are two coatings having equivalent adhesion strength to a substrate but different surface slickness, the slicker one is more resistant to scrubbing.